Thermoelectric cooling systems are analogous to conventional refrigeration cooling systems. For example, a conventional cooling system includes an evaporator, a compressor, and a condenser. In the evaporator or cold section, pressurized refrigerant is allowed to expand, boil, and evaporate. During the change of state from a liquid to a gas, energy in the form of heat is absorbed. In the next step, the compressor recompresses the gas into a liquid. Further, the condenser expels the heat absorbed at the evaporate and the extra heat added by the compressor to the ambient environment.
A thermoelectric cooling system has similar subassemblies. However, thermoelectric cooling is specifically the abstraction of heat from electronic components by Peltier effect, greatly improved and made practicable with solid-state thermoelectric materials, e.g., Bi.sub.2 Te.sub.3. Devices using this effect, e.g. frigistors, are used for automatic temperature control, and the like and are energized by direct current ("d.c.") thermoelectric materials, that is, any set of materials (metals) which constitute a thermoelectric system. Some examples include: "binary" systems (bismuth and tellurium), ternary systems (silver, antimony and tellurium), and "quaternary" systems (bismuth, tellurium, selenium and antimony, called "Neelium"). The Peltier effect is a phenomenon whereby heat is liberated or absorbed at a junction when current passes from one metal to another. In this application, a cold junction (the place where the heat source or load is located) is defined as the assembly where energy in the form of heat is absorbed when current passes from one metal to another. A hot junction (the place where the heat sink is located) is the assembly which thermally communicates with a heat exchanger and through which the heat that is liberated, when current passes from one metal to another, is transferred to the ambient environment.
Major differences exist between thermoelectric cooling systems and conventional refrigeration systems, however. For example, conventional refrigeration systems must maintain a dosed environment isolated from the ambient. Further, conventional refrigeration systems have a large amount of insulation and cannot be ventilated without loss of cooling effect. Thus, conventional cooling systems may contain odors of the articles placed within and such odors may be transferred to other articles placed within the cooling system, with obviously undesirable results. Further, conventional cooling systems produce humidity which may adversely affect the physical characteristics of the product being cooled, such as texture, taste, shelf life, and the like, of certain food articles which may be placed therein. For example, fresh baked bread may, if humidity and temperature are not carefully controlled, become soggy on at least one side during the cooling process.
Thermoelectric cooling systems, by contrast, provide a measure of advantage to the several shortcomings noted above. However, thermoelectric cooling systems of the prior art lack efficiency in certain respects because, upon interruption of the power supply, the current reverses flow such that what was a heat source becomes the heat sink, and what was the heat sink now becomes the heat source.
It is an objective of the present invention to provide a thermoelectric cooling circuit that substantially prevents reversal of the heat source and heat sink when power is substantially interrupted upon a predetermined temperature range being reached at the heat source. It is an additional objective of the present invention to provide a bread box and a wine cooling rack using the thermoelectric cooling circuit of the present invention.